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10 June 2014, 19:25 | Updated: 10 June 2014, 19:34
Mayor Boris Johnson's bought three water cannon for London, without being given government permission to use them.
Around £90,000 is being spent buying the cannon from German police, and another £40,000 transporting and refitting them.
But the Mayor's office has argued that around £2.4 million is being saved by buying them second-hand now, rather than waiting for the Home Secretary to make her decision.
An independent panel will look at when water cannon should be used, but central government funding for them has already been ruled out.
Labour and the Liberal Democrats condemned the action as "reckless", saying there's not enough evidence that the tactic is effective in keeping order.
Cannon are generally used to keep control during riots, but have never been used on British mainland before.